1. Field of the Invention
The present invention relates generally to an image forming apparatus and, more particularly, to a system and method for determining electrical and geometrical parameters of a transfer nip for transferring toner in an electrophotographic system.
2. Description of the Related Art
This invention concerns the transfer process for electrophotographic printers. It applies to both two step transfer and direct-to-paper imaging systems. Specifically it applies to the transfer process, whereby toner is moved from a donating medium, such as a transfer belt, to an accepting medium, such as a sheet of paper or transparency.
Transfer is a core process in an electrophotographic printing process. The process starts when a photosensitive roll, such as a photoconductor, is charged and then selectively discharged to create a charge image. The charge image is developed by a developer roll covered with charged toner of uniform thickness. This developed image then travels to what is referred to as “first transfer” in the case of a two step transfer system, or the only transfer process in the case of direct-to-paper systems.
In either system, the toner enters a transfer nip area between a photoconductor roll and a transfer roll. The media to which the developed toner image is to be transferred, either a transfer belt for a two step transfer system or a transport belt supporting paper for a direct-to-paper system, is positioned between these two rolls. Time, pressure and electric fields all influence the quality of the transfer process. A voltage is applied to the transfer roll to create a field to pull charged toner off the photoconductor onto the desired medium.
In a two step transfer system, the transfer belt, now carrying the charged toner, travels to a second transfer nip, similar in some ways to the first transfer nip. The toner is again brought into contact with the toner receiving medium in the second transfer nip formed by a number of rolls. Typically a conductive backup roll and a resistive transfer roll together form the two primary sides of the second transfer nip. As with the first transfer, time, pressure and applied fields play significant roles in ensuring high efficiency transfer.
Transfer robustness is frequently measured as the amount of voltage between the lowest voltage at which acceptable transfer occurs due to a sufficient electric field having been established to move toner, and the highest voltage at which acceptable printing occurs before Paschen breakdown, i.e., the voltage at which the dielectric properties of the materials in the transfer nip begin to break down, causes undesirable print artifacts. This robustness varies across environments as the properties of the receiving media vary over those same environments. The larger the difference between the lowest and highest voltages, the more tolerance exists for part-to-part variation while still yielding relatively good quality prints.
The low end of the transfer window is typically determined by how well the electric field, measured in volts/meter, can be established, and by how much electric field is then required to overcome the forces of adhesion between the toner and the donating medium. The high end of the transfer window is the point at which the electric field established to transfer the toner exceeds the Paschen breakdown limit, allowing a discharge event to happen. Depending on the location of the breakdown, various print defects will be present in the page which would make the print unacceptable.
An ongoing demand exists in the printer industry for a faster, more versatile printer that provides higher print quality. Process speeds for printers have steadily increased and the present goal is to have smaller, stand-alone printers which can deliver high print quality at speeds once reserved for large printing presses. In addition to faster speed requirements, the demands for smaller sizes of these systems mean that they can be more readily placed in less climate-controlled locations while maintaining their high quality output. Changes in temperature and relative humidity have been seen to have a relatively sizeable impact on the electrical properties of the media on which the systems print.
For instance, recent volume resistivity measurements of a variety of common printer media over a class B range of environmental conditions have shown a shift in volume resistivity equal to about eight orders of magnitude over the range measured.
In addition to changes in resistance due to temperature and moisture content, paper resistance is also strongly influenced by the electrical field placed across the sheet. While a conventional resistor behaves according to Ohm's Law, paper resistance changes with the applied voltage field. This non-ohmic behavior is a function of a charge separation that takes place inside the media in response to any externally applied electrical field. For instance, for bond paper at 60 degrees C. and 8% relative humidity, a drop in resistance of over 75% has been seen in response to a change in voltage across the bond paper from about 500 v to about 1500 v.
A further complication with the electrical properties of paper is that the amount of time for charge separation to occur is a function of the material resistance, which is changing both in environment and by the applied electric field. The chart of FIG. 1 shows an approximate voltage versus time response for Hammermill® Laser 24# paper at two different environments: 72 degrees F., 50% relative humidity; and 60 degrees F., 8% relative humidity.
What may be the most significant complication concerning paper's electrical properties is that the dielectric breakdown strength thereof is seen to be relatively strongly influenced by the environment. The electric field that a paper can withstand when dry is seen to be significantly higher than the electric field the same paper can support when in a humid environment. For an example, Strathmore bond writing paper, 24#, has a dielectric breakdown strength of about 1780 volts at 60 degrees F. and 8% relative humidity, but at 78 degrees F. and 80% relative humidity the breakdown strength is only about 400 volts, which is less than 25% of the corresponding dry value. Other paper has been seen to respond similarly.
As described above, paper and a toner-covered donating medium (belt or photoconductor) enter the transfer nip where an electric field and pressure cause the toner to transfer from the donating medium to the paper. The length of time in the nip affects how quickly the electric field must be established such that there is a sufficient Lorenz force to cause the toner to move. The faster the process speed of the printer, the less time the paper and other nip components have to respond and create a good situation for toner transfer.
Traditionally, the time constant of the transfer nip is controlled by the transfer roller, and the resistance of the foam for that roller is chosen to be appropriate for the speed of the printer and therefore the time in the nip. If the printer is to operate at faster speeds, the resistivity of the roll is decreased in order to provide for an appropriate time constant to meet the new process speed requirements.
Unfortunately, as the resistivity of the transfer roll drops the onset of over transfer defects occurs at lower voltages than the corresponding decrease in the voltage required to achieve good transfer. In other words, the robustness of the system, as measured by the voltage width of the transfer window, decreases. Defects associated with this decrease in robustness include a speckling defect caused by electrical breakdown of paper in the transfer nip when the paper is dry. When the paper is more humid the over transfer defect is more spread out and a fading of low charge areas like half tones or low charge solid areas result. Both of these defects result in reduced print quality for the printer user.
Based upon the foregoing, there is a need for an improved transfer nip in an electrophotographic imaging system.